The invention is related to the field of optical isolators, and in particular to a slab coupled optical waveguide to form an optical isolator, where the slab coupled optical waveguide includes a magneto-optic material to achieve nonreciprocal loss.
Since the invention of diode lasers in 1962, researchers have sought to increase the power available from them in a single spatial mode. Because of their high brightness, single-mode diode lasers have many advantages for a variety of applications. Single-mode, high-power diode lasers are used to pump erbium-doped fiber amplifiers that are essential to fiber optical communications. Others pump fiber lasers in a number of commercial and industrial applications, but the amount of pump light that can be coupled into the fiber is generally limited by the diode's low beam quality.
In another example, diode lasers are of interest for being used directly in materials processing, and their efficacy depends on their brightness because the beam intensity on a distant work piece can be dramatically increased by increasing the source's brightness. Brightness is also important for diode lasers in free-space optical communications because the fractional amount of the transmitter's light that reaches the receiver is proportional to brightness.
Numerous methods have been employed to boost the single-mode output power from a diode laser. Although the power that is available from conventional single-mode ridge lasers keeps rising, these increases have been incremental, and optical loss and heat removal impose definite limitations. Moreover, ridge-waveguide lasers usually have highly elliptical beams that diverge much more rapidly in one axis than in the other. External optics are necessary to couple these beams into a single-mode fiber.
Tapered lasers have been developed to increase the mode in the lateral direction while keeping the device in a single mode. These lasers use an adiabatic taper along their length to enlarge the optical mode, but these structures suffer from beam instability problems at high powers, and heroic optics are needed to couple the astigmatic output beam to a fiber.
In a slab coupled optical waveguide laser (SCOWL), the beam is expanded in the transverse (Y) direction, perpendicular to the plane of the device, such that the fast and slow directions of the laser mode can be made nearly equal in dimension. A thin gain region carefully placed within a large, low-loss, passive waveguide results in a low modal loss, which allows the construction of longer devices, spreading out the heat and reducing series resistance. The net result is high power in large, nearly circular, diffraction-limited mode. This mode profile is ideally suited, for example, for efficient coupling into a single-mode fiber. The brightness (B) of a laser is expressed by:B=P/(λ2Mx2My2),where P is the power emitted from the laser, λ is the laser wavelength and Mx2 and My2 are the beam quality parameters in the vertical and horizontal directions, respectively. Clearly, brightness is highest for the lowest mode order. Brightness is the metric that measures, for example, the amount of power that can be delivered on a far-field target. The differences in brightness between multimode and single-mode lasers can be large; e.g., a typical multimode pump laser with P=1 W, λ=1 μm, Mx2=1, and My2=20 has B=5 MW/cm2-sr. On the other hand, a single-mode laser—e.g., a SCOWL—with P=1 W, λ=1 μm, and Mx2=My2=1 has B=100 MW/cm2-sr. The multimode laser would have to be 20 times more powerful than the single-mode one to produce equivalent brightness.
The SCOWL uses the concept of slab coupling in which high-order modes of a large waveguide are filtered out by coupling to slab modes, thereby transforming this multimode waveguide into a single-mode one. More than 30 years ago, Enrique A. J. Marcatili used the coupled-mode theory to show that an arbitrarily large slab-coupled passive waveguide should be possible. Marcatili demonstrated that a large, round fiber can propagate many modes, but when the fiber is brought close to the slab, its modes couple to those of the slab. If only the lowest-order fiber mode has a propagation constant higher than the propagation constants of the slab mode (βz-slab in the figure), the high-order fiber modes will couple to the slab, leaving the composite structure with only one bound mode; that is, the composite structure will be single-moded.
Marcatili's analysis is straightforward, but reality is more complicated. Even though there is only one true bound mode, other slowly decaying, or “leaky,” modes can exist. These can be problematic in lasers, but they are often an issue even in purely passive guides because they can be inadvertently excited at the input, and it takes a long distance for them to radiate away. Propagation in curved waveguides really occurs via leaky modes, as does propagation in AlGaAs waveguides on GaAs substrates, because the substrate has an index greater than or equal to that of the guide.
For good single-mode operation in a slab-coupled guide, all potential leaky modes should couple efficiently into the slab so that they will radiate away quickly. If they don't couple efficiently and their coupling loss is less than their gain, they will reach threshold and oscillate. For the leaky modes to be sufficiently lossy, they must be well-coupled to one or more slab modes that have higher propagation constants